DNA replication occurs in both directions away from the origin. Both strands are synthesized at each replication fork by DNA polymerase III holoenzyme: one of the alpha polymerase subunits of the holoenzyme synthesizes the leading strand (blue) continuously in the direction of fork movement while the other alpha polymerase subunit of the holoenzyme synthesizes the lagging strand (red) discontinuously as Okazaki fragments in the opposite direction of fork movement. Okazaki fragments have about 10 RNA nucleotides at the 5' end (RNA primer) and are about 1000 DNA nucleotides long. The RNA is subsequently removed from Okazaki fragments by DNA polymerase I and the fragments are then joined together by DNA ligase into long, continuous DNA. The arrows point in the direction of DNA synthesis (5'-to-3').